On Tue, 4 Jan 2011 19:28:35 -0800 (PST), "capt.bill11"
wrote:

Looking at building a hard top here in Costa Rica.

One idea I had was to build the frame/skeleton of the top out of
aluminum square tube welded together to form a grid. And then gluing
foam to the top, bottom, sides, shape it, skin it with fiberglass,
fair, prime and paint it. The wire chases would be PVC pipe.

So my questions are,

Is it a stupid idea?

If not, what kind of adhesive would be best to use to attach the foam
to the aluminum grid?

Is there a better material to make the grid out of?

What kind of foam board would be best to use for something like this?

How would one go about calculating the maximum cantilever that a
structure like that could span?

Thanks

You are on the right track. A good friend (The Fiberglass Guy) has
built several of these Foam Biminies" for want of a better
description.

First of all the type of foam you are using determines how much
framing you need. In the Fiberglass Guy's case he is using
"construction foam, i.e. Divinycell (spelling?) 80 Kg./M3 in 4' X 7'
sheets.

He builds a form and then slices the foam in strips and lays it down
on the form to build an arched top. Glues the strips and fills the
cracks between the strips with filler. He then removes from the form
and adds the edges, usually 3 - 4" strips glued to the arched top to
form a edge all the way round the top. Then whatever hard points, of
Marine grade ply are inlayed. Fill and sand smooth and then lays on
two (I think) layers of 600 gm. biaxial cloth, inside and out. If the
finish is to be gelcoat then polyester resin, if paint then epoxy.

The supporting frame is just a stainless square with mounting legs
that the top is mounted to with bolts and clamps.

He is usually building as a cockpit top for say 35 ft., or perhaps a
bit larger, cats, but has built at least one as a cockpit top for a 35
ft. mono hull

The reason for the high end foam is that these "biminies" are
essentially self supporting and the frame essentially just holds the
thing up there.

By the way, I have built several small structures from 1" Styrofoam
covered with one layer of 400 gm. cloth. They are amazingly strong,
considering the materials. However do have the down side that they
aren't very resistant to crushing. I add this comment as I'm really
not sure how dense a foam is actually needed. My mate is building
commercially and doesn't want the thing to break ( for the price he
gets it better not break!) so a cheaper foam probably could be used.

Cheers,

Bruce
(bruceinbangkokatgmaildotcom)